The present invention is directed to heat exchangers of the type utilizing a liquid to effect a transfer of heat to another liquid and the vaporization of the second liquid.
When a heat exchanger is employed as an evaporator or boiler, as distinguished from a condenser, it is important to keep all surfaces wetted so that a maximum heat transfer effect occurs between the wetted surfaces and the liquid in order to create bubbles or boiling by the vapor action. As is well known in the field of refrigeration, when an evaporator is covered or filled with a liquid which is boiled by having heat applied to it through the heat exchanger surfaces, then the liquid itself has much higher density, usually, than the vapor which is created by boiling. If a vessel or chamber is filled with this liquid, obviously the weight of the liquid creates a higher pressure at the bottom of the chamber than at the top. This affects the saturation pressure or saturation temperature of the liquid. It is known that where pressure is higher a liquid must be raised to a higher temperature to make it boil. Thus, in a vessel or chamber containing liquid if you try to boil same the temperature must be raised to a higher level at the bottom of the column of liquid within said vessel or chamber than at the top of said column of liquid. This creates a heat transfer penalty, since the temperature difference between the water used to boil the fluid and the fluid at boiling temperature, is different at the bottom and the top. In common parlance this is called the "static head penalty" in an evaporator structure. In order to reduce this static head penalty it is advisable to keep the liquid level flooding the evaporator as low as possible so that there is as little difference between the top and the bottom of the column of liquid as possible. This then would reduce the static head penalty and improve the available temperature difference for creating boiling.